1. Field of the Invention
This invention relates to a treatment of unreacted synthesis gas (syngas) produced in a gas-to-liquid synthesis and more particularly to a process of contacting unreacted syngas with water to remove CO2.
2. Description of Related Art
There is a considerable economic incentive to exploit the production of natural gas which is an abundant resource normally available only at remote sites. Frequently, it is not economically viable to transport natural gas from such remote sites to commercial markets or off-site processing facilities. One approach has been to convert the natural gas into liquified natural gas (LNG) for transport to markets or processing facilities. Another approach has involved converting natural gas into methanol at the remote site without further processing of methanol into gasoline.
Natural gas is a primary source of methane which is used to manufacture synthesis gas. Synthesis gas (syngas) is primarily a mixture composed of CO and H2. Techniques are known to convert syngas into useful products such as methanol or into synthetic fuels, lubricants and other hydrocarbonaceous products via Fischer-Tropsch synthesis. One method for the preparation of syngas involves catalytically reacting methane and carbon dioxide. While natural gas is a primary source of methane, coal and petroleum have also been utilized to provide hydrocarbon feeds to generate syngas.
Carbon dioxide is considered by some to be a major factor in global warming. Accordingly, there is an incentive to find means for reducing the production of carbon dioxide and limiting its release into the environment. One advantage of the aforementioned process for preparing syngas is that it utilizes carbon dioxide as a reactant.
At present, there are two gas-to-liquid (GTL) technologies which convert remote natural gas assets or coal into transportation fuels and lubricants. Both use syngas as an intermediate. The first involves the conversion of natural gas or coal into syngas by partial oxidation followed by reaction in a Fischer-Tropsch synthesis with further refining of the Fischer-Tropsch products. The second technology involves conversion of natural gas or coal into syngas by partial oxidation followed by methanol synthesis, the methanol being subsequently converted into highly aromatic gasoline by the Methanol-To-Gasoline (MTG) process.
The Fischer-Tropsch and MTG processes both have relative merits and disadvantages. One advantage of the Fischer-Tropsch process is that the products formed are highly paraffinic. These products have excellent combustion and lubricating properties. A disadvantage of the Fischer-Tropsch process is the relatively large amounts of carbon dioxide that are emitted in the facility during the conversion of natural gas into Fischer-Tropsch products. The MTG process produces a highly aromatic gasoline and LPG fraction. While the gasoline generally is suitable for use in gasoline engines, durene and other polymethyl aromatics may be present. These materials have high crystallization temperatures and can solidify upon standing. The MTG process also suffers from higher capital costs in comparison to the Fischer-Tropsch process and the product cannot be used for lubricants, diesel fuel or jet turbine fuel.
A typical Fischer-Tropsch process is illustrated in FIG. 1. A feed of CH4, O2 and H2O is forwarded via conduit (10) to a syngas generator (15). Effluent from the generator containing CO, H2 and CO2 is forwarded via conduit (20) to a Fischer-Tropsch reactor (25). The products of the reaction are forwarded via conduit (35) to a separation zone (40). Hydrocarbonaceous products including C5+ liquids are recovered and forwarded via conduit (45) to other areas of the facility for further processing into fuels, lubes, etc. Gaseous products recovered from the separation zone (e.g. tail gas) include CO, H2 and CO2. A portion of the tail gas is forwarded via conduit (60) for use as a fuel in the facility. Another portion of the tail gas is recycled via conduit (50) to be mixed with the feed to the syngas generator (15).
The origin of the CO2 emissions from the Fischer-Tropsch synthesis can best be understood by examining the stoichiometry of the reaction. The major products of a Fischer-Tropsch reaction are paraffins and olefins, and these can be represented by the formula nCH2 which represents a paraffinic polymer of n CH2 units This formula is exact for mono-olefins and a close approximation for C5+ paraffins. The value of n (the average carbon number of the product) is determined by the reaction conditions, e.g., temperature, pressure, space rate, catalyst type, and syngas composition. The desired net syngas stoichiometry for a Fischer-Tropsch reaction is independent of n, and is approximately 2.0 as determined by the following equation:
nCO+2nH2xe2x86x92nH2O+nCH2
where nCH2 represent the major products of a Fischer-Tropsch reaction (olefins and paraffins).
There are three general reactions that produce syngas from CH4. These are:
Steam reforming of CH4:
CH4+H2Oxe2x86x92CO+3H2
However, the ratio of H2 to CO is 3:1 which is higher than the 2:1 ratio desired for the Fischer-Tropsch conversion.
Dry reforming, or reaction between CO2 and CH4:
CH4+CO2xe2x86x922CO+2H2
However, the ratio of H2 to CO is 1:1, which is lower than that desired for the Fischer-Tropsch conversion. Also, dry reforming may result in rapid carbon deposition.
Partial oxidation using O2:
CH4+xc2xdO2xe2x86x92CO+2H2.
This provides the desired 2:1 ratio of CO and H2 and is the reaction that is to be emphasized.
In commercial practice, an amount of steam is added to a partial oxidation reformer in order to control carbon formation. Likewise, some CO2 can be tolerated in the feed. So while partial oxidation is the emphasized reaction, all reactions occur to some extent in the reformer.
CO2 is formed in partial oxidation because the reaction is not perfectly selective. Some CH4 reacts with O2 to form CO2 by complete combustion according to the following:
CH4+O2 xe2x86x92CO2+2H2
and
CH4+2O2xe2x86x92CO2+2 H2O
Furthermore, steam added to the reformer to control coking, or produced in the Fischer-Tropsch reaction, can react with CO to form CO2 by the water gas shift reaction as follows:
CO+H2Oxe2x86x92CO2+H2
This reaction reaches equilibrium, and the reverse of it is known as the reverse water gas shift reaction:
CO2+H2xe2x86x92CO+H2O
Furthermore, light by-product gases, which include C1-C4 hydrocarbons, are frequently used as fuel in furnaces. This fuel often includes the CO2 from the GTL facility along with some unreacted CO. The furnaces provide the heat to the process, and contribute significant amounts of CO2. With Fischer-Tropsch catalysts that do not promote the water gas shift reaction (Co-based catalyst rather than Fe-based catalysts), and with proper operation of the reformer and other units, the major source of CO2 is combustion of hydrocarbons in the furnaces.
Thus, a significant amount of CO2 is formed during the conversion of CH4 into transportation fuels and lubricants by the Fischer-Tropsch process. This CO2 exits the GTL-Fischer-Tropsch process in the tail gas from the Fischer-Tropsch unit, i.e., in the gases that are not consumed in the Fischer-Tropsch reactor.
The overall proportion of carbon in the CH4 that is converted to heavier hydrocarbon products is estimated to be about 68%. The remainder, about 32%, forms significant amounts of CO2. These estimates of carbon efficiency were provided by Bechtel Corporation for a GTL complex that uses cryogenic air separation, an autothermal reformer, a slurry bed Fischer-Tropsch unit and a hydrocracker for conversion of the heavy wax into products. Details are described in xe2x80x9cCO2 Abatement in GTL Plant: Fischer-Tropsch Synthesis,xe2x80x9d Report # PH3/15, November 2000, published by the IEA Greenhouse Gas RandD Programme. GTL complexes using alternative technologies would have similar carbon efficiencies and CO2 emissions.
To control the reaction, syngas conversion processes operate at less than 100% conversion of the CO in the syngas. Typical values are between 40 and 70% per-pass conversion. In Fischer-Tropsch processes which use O2 rather than air, the unreacted syngas is recycled to the Fischer-Tropsch reactor. From the standpoint of economics and operational efficiency, the preferred Fischer-Tropsch process is a slurry bed process. Also, the most common catalyst for use in slurry bed units contains cobalt. Cobalt does not promote the water gas shift reaction to a significant extent (or the reverse of this reaction). In these units, CO2 for the most part, is an inert gas. As CO2 is recycled to the syngas conversion reactor, its concentration builds up. This effectively lowers the concentration of the reactive syngas components (CO+H2), and reduces the rate of reaction. To compensate for the lower partial pressures of the reactive components, the pressure of the Fischer-Tropsch reactor is increased. During this recycle operation, a small amount of CO2 in the initial syngas (typically 5 vol % but always 2% or more) increases to much larger values (typically 40 vol %). In commercial practice, typically a portion of the CO2-enriched recycle gas is recycled to the syngas formation reactor where it promotes the dry reforming reaction and reduces the ratio of H2 to CO in the syngas to the desired level. The recycle of CO2 to the syngas generator reduces the selectivity for CO2 formation and improves the selectivity for formation of the desirable syngas components CO and H2. This is because CO2 is produced in equilibrium with CO, H2 and H2O due to the water gas shift reaction. However, much more CO2 is produced than can be consumed in the syngas generator, and the excess CO2-enriched recycle gas is purged from the process and used as a low energy content fuel. The use of this low energy content fuel is a significant source of the CO2 emissions from the GTL facility.
A process scheme which reduces the CO2 emissions from a Fischer-Tropsch-GTL process while still making the desired product slate would be highly desirable. Reducing the CO2 emissions also acts to improve the carbon efficiency of the process.
EP0 516 441 A1 discusses several aspects of handling CO2 in a Fischer-Tropsch-GTL process including: separating CO2 from the syngas generated in the reformer, and recycling the unreacted tail gas from Fischer-Tropsch to the reformer. As discussed previously, the recycling of CO2 is to achieve proper H2/CO ratio of syngas. This publication notes that: xe2x80x9cSeparation of carbon dioxide is expensive. It is often carried out by amine stripping. This involves reaction with an amine, followed by boiling, and compression to reach the necessary pressure for recycle back to the reformer.xe2x80x9d (page 3, lines 4-6). Likewise: xe2x80x9cThe cost of removing and recycling the CO2 is expensive and typically would represent around 30% of the costs associated with producing the syngas within the process.xe2x80x9d (page 4, lines 45-46).
EP 142 887 B1 discusses the benefits of having CO2 in the feed to the reformer. It also mentions the high costs associated with the separation of the CO2 from the syngas by amine scrubbing and subsequent compression.
Neither of these publications disclose the use of an aqueous medium to remove CO2 from syngas. The use of an aqueous medium to remove CO2 significantly reduces costs associated with typical amine scrubbing, and also provides a source of water for use in the reforming reaction.
It is an object of the invention to provide an efficient process which reduces the cost of separating CO2 in recycled synthesis gas.
It is another object of the invention to develop a technique which lowers the CO2 emissions from a GTL facility.
These and other objects and advantages of the present invention will become apparent to the skilled artisan upon a review of the following description, the claims appended thereto, and the figures of the drawings.
These and other objectives of the invention are attained by a process which includes the steps of:
A process comprising the following steps:
(a) forming a syngas which contains CO2 in a syngas generator;
(b) reacting the syngas in a syngas conversion process to form a product stream comprising hydrocarbonaceous products and a tail gas containing unreacted syngas and CO2;
(c) separating the hydrocarbonaceous products from the unreacted syngas and CO2;
(d) contacting at least a portion of the unreacted syngas and CO2 with an aqueous medium having a pH above about 6.0 in a scrubbing zone to adsorb at least a portion of the CO2, and recovering a CO2-enriched aqueous stream and a syngas with reduced CO2 concentration;
(e) forwarding at least part of the recovered syngas from step (d) to the syngas conversion reactor; and
(f) desorbing at least part of the CO2 from the CO2-enriched aqueous stream obtained in step (d) and recovering a CO2-rich gas and CO2-depleted aqueous stream.
Desorption can be accomplished, for example, by contacting the CO2-enriched aqueous stream with CH4 in a desorption vessel. Recovered CH4 can be recycled to the syngas generator. Portions of the recovered CO2 can be disposed in a marine environment, a terrestrial formation or both.
The recycling of CO2 and CH4 to the syngas formation reactor increases the carbon efficiency of the process. The separation of CO and H2 and the processing of these gases in the syngas conversion unit thereby avoids forwarding them to the syngas formation reactor. This reduces the total amount of gases processed in the syngas formation reactor while shifting the equilibrium towards the desired CO and H2.